In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and other raw materials may be converted to solid polyolefin product. Generally, gas phase reactors may include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction may be maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. Catalyst is added to the fluidized bed. Heat of reaction may be transferred to the circulating gas stream. This gas stream may be compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
The properties of the polymer formed by such a process can be controlled to some extent by varying the operating conditions, including the operating temperature, comonomer amount, and type and quantity of catalyst. Such properties include the molecular weight of the polymer product, the molecular weight distribution of the polymer product, polymer density, and the flow index of the polymer product. The properties of the polymer product as extracted from the reactor system, as well as in processed form for sale to customers, may also important. Typically, polymer product is extracted from the reactor and extruded into a more manageable form, such as pellets or bars. Therefore, it would be desirable to control the molecular weight of the polymer after extraction from the reactor to some extent.